In ink jet printing technology, the main concerns are to improve the quality as well as the speed of printing.
Printing technologies developed today almost all have the objective of producing high quality copies as fast as possible. In the case of ink jet technologies, to achieve fast printing, the various manufacturers multiply, on the surface of the heads, the number of nozzles that are capable of ejecting ink drops in order to print a greater number of points in parallel on the receiving medium.
One technology for making ink jet printing heads consists in forming and ejecting ink drops by the action of a pressure wave created by the pulsation of a piezoelectric ceramic.
Another conventional technology for making ink jet heads consists in raising to a high temperature, typically 300 to 400.degree. C., the ink located in a channel for a very short time. This induces local vaporization of the ink which expulses that liquid part of the ink located between the vaporization zone and the surface of the ink jet head in the form of a drop. This method requires thermal energy within the volume of the ink jet head itself, which then has to be dissipated.
Another technique, for example as described in patent application WO96/32284, consist in bringing a fluid into contact with a ring shaped heating element located at the periphery of the aperture of a channel linking a reservoir containing fluid with the aperture at the surface of the ink jet head. Pressure is applied to the reservoir in order to enable the ink to be conveyed through the channel and to be spread on the ring shaped heating surface of the ink jet head. When the heating element of the ink jet head is raised to a temperature of about 130.degree. C., a significant modification takes place to the surface tension of the ink drop located in contact with the heating element. This surface tension modification causes a reduction in the radius of curvature of the meniscus of the ink drop thus enabling it to flow freely through the channel and to form a drop of suitable size for the printing required. Once formed, this drop is then ejected by a means which may be an electrostatic field between the ink jet head and the printing medium, for example a sheet of paper. This technique, which has the advantage of considerably lowering the temperature necessary to eject a unit volume of ink, is thus appropriate for the manufacture of highly integrated ink jet heads.
Ink jet printing heads are capable of delivering several thousand ink drops with a unit volume of some picoliters per second. These heads have increasingly small dimensions. They are produced by micro-engineering or micro-manufacturing techniques. With such devices, the control of the ink quality becomes a critical factor. In fact, variations in ink properties can affect the operation of the heads, for example by gumming them up, and the printing quality. The inks are complex mixtures, in the form of dispersions, of emulsions or solutions of dyes or pigments in solvents, in water based or mixed media. These mixtures contain many additives such as antifoaming agents, agents for facilitating grinding, surfactants, biocides, buffers, thickeners. These mixtures, to be suitable for ink jet printing, must have a set of characteristics which are: pen reliability, i.e. resistance to polymerization, stability to oxidation or the action of bacteria, and ejection capability, i.e. the capacity to form drops that can be ejected by the printing head. These characteristics are described by H. J. Spinelli in Advanced Materials, 10, No 15, pages 1215-1218 (1998). Thus, the inks and the dyes they contain are sensitive to air oxidation. This oxidation modifies the ink properties, and thus the process of forming drops and affects the printing performance. According to the prior art, attempts to remedy ink instability to oxidation have been made by adding to these inks antioxidants such as dithionous acid, sodium sulfite, pyrogallol, sulfites or ascorbic acid, as is described in U.S. Pat. Nos. 4,489,334 or 4,279,653, or in Japanese Patent Applications 79/98690 or 79/116710. However, the action of these antioxidants remains limited and it complicates the ink manufacturing process. It is also known that the presence of air bubbles in the ink disturbs both the ink flow rate to the printing head and the formation of ink drops. Thus, it is desirable to control the atmosphere in ink reservoirs to prevent bubbles forming and then being conveyed in the printer channels to the printing head. It can also happen that air enters when the ink reservoir or printer is filled, or when the ink cartridge is changed, if interchangeable cartridges are used. According to the prior art, attempts have been made to remedy this problem by equipping ink reservoirs or printer cartridges with valves to prevent the introduction of air in the ink circuit to the print heads, as is described in U.S. Pat. No. 5,812,155. The utilization of these valves introduces an undesirable complexity into the technology of ink jet printers and associated reservoirs.